<meta charset="utf-8">
(#) `String.format` string only contains trivial conversions

!!! WARNING: `String.format` string only contains trivial conversions
   This is a warning.

Id
:   `StringFormatTrivial`
Summary
:   `String.format` string only contains trivial conversions
Note
:   **This issue is disabled by default**; use `--enable StringFormatTrivial`
Severity
:   Warning
Category
:   Performance
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   7.3.0 (September 2022)
Affects
:   Kotlin and Java files and resource files
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/StringFormatDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/StringFormatDetectorTest.java)

Every call to `String.format` creates a new `Formatter` instance, which
will decrease the performance of your app. `String.format` should only
be used when necessary--if the formatted string contains only trivial
conversions (e.g. `b`, `s`, `c`) and there are no translation concerns,
it will be more efficient to replace them and concatenate with `+`.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/Trivial.java:10:Warning: This formatting string is trivial.
Rather than using String.format to create your String, it will be more
performant to concatenate your arguments with +.  [StringFormatTrivial]
    String output1a = String.format("%s", "Hello world");
                                    ----
src/test/pkg/Trivial.java:11:Warning: This formatting string is trivial.
Rather than using String.format to create your String, it will be more
performant to concatenate your arguments with +.  [StringFormatTrivial]
    String output2a = String.format("%s %c", "Hello world", '!');
                                    -------
src/test/pkg/Trivial.java:12:Warning: This formatting string is trivial.
Rather than using String.format to create your String, it will be more
performant to concatenate your arguments with +.  [StringFormatTrivial]
    String output3a = String.format("%s %c %b", "Hello world", '!', true);
                                    ----------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are the relevant source files:

`src/test/pkg/Trivial.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import android.content.res.Resources;

public class Trivial {
    private static void test(Resources resources) {
        String trivial1 = resources.getString(R.string.trivial1); // OK
        String trivial2 = resources.getString(R.string.trivial2); // OK
        String trivial3 = resources.getString(R.string.trivial3); // OK
        String output1a = String.format("%s", "Hello world");
        String output2a = String.format("%s %c", "Hello world", '!');
        String output3a = String.format("%s %c %b", "Hello world", '!', true);
        String output1b = String.format(trivial1, "Hello world");
        String output2b = String.format(trivial2, "Hello world", '!');
        String output3b = String.format(trivial3, "Hello world", '!', true);
    }

    public static final class R {
        public static final class string {
            public static final int trivial1 = 0x7f0a0000;
            public static final int trivial2 = 0x7f0a0001;
            public static final int trivial3 = 0x7f0a0002;
        }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`res/values/strings.xml`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~xml linenumbers
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;resources&gt;
    &lt;string name="trivial1"&gt;%s&lt;/string&gt;
    &lt;string name="trivial2"&gt;%s %c&lt;/string&gt;
    &lt;string name="trivial3"&gt;%s %c %b&lt;/string&gt;
&lt;/resources&gt;

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/StringFormatDetectorTest.java)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `StringFormatDetector.testTrivialString`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("StringFormatTrivial")
  fun method() {
     format(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("StringFormatTrivial")
  void method() {
     format(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection StringFormatTrivial
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="StringFormatTrivial" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'StringFormatTrivial'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore StringFormatTrivial ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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